The invention relates to electronic telephone stations. It relates in particular to a circuit for surge voltage protection in such stations, where the feed voltage is supplied via a two-wire subscriber line.
A known circuit of this kind includes a varistor, a polarity reversal protection device, a current limiting device, a transistor circuit, and a Zener diode. The varistor shunts the inputs of the circuit, and the polarity reversal protection device is connected in parallel with the varistor. That terminal of the polarity reversal protection device, which conducts the positive potential of the feed voltage, is connected with a first transistor of the current limiting device. If this transistor is, e.g. a V-MOS (or bipolar) transistor, it can also be used as a so-called "Dialswitch-Pulsing" contact. It is then driven--via a transistor circuit consisting of two additional transistors and biassing resistors--by the dialing pulse generated in a dialing keyboard. The Zener diode is connected in parallel with the actual station circuit comprising the essential electronic assemblies.
Both the Zener diode and the current limiting device, which could be connected in different ways between the polarity reversal protection device (e.g. a full-wave rectifier bridge) and the station circuit, protect the station against surge voltages. The current limiting device limits the input currents which can differ in magnitude on account of subscriber lines of unequal length, up to a maximum current for the station circuit. The varistor preceding the circuit arrangement in known manner provides an additional protection against temporary high voltages on the subscriber line which may be induced by external influences such as, for example, lightning.
However, the current limiting device can have relatively high tolerances and these tolerances are a function of the maximum feed current flowing to the station circuit. Thus, when high voltages (lightning) are input-coupled onto the subscriber line while the varistor is turned on, impermissibly high power losses can occur both across the V-MOS-transistor and across the Zener diode. In order to prevent their destruction or damage, components must be provided which can tolerate such power losses and are thus expensive.
It is therefore an object of this invention to provide a circuit for protecting a line-powered telephone station from surge voltages using components which need not be especially resistant to power losses but are nonetheless protected from destruction in the event of temporarily high surge voltages.
It is a further object of this invention to improve on devices of the art.